Hi Naetha
The first thing that jumped out at me was that you 'asked your bosses for a development plan'.It may just have been the way you phrased it, but I would always be much more impressed with someone who came to me with a development plan already written, with objectives in terms of promotion etc, identifying what skills/experience need to be developed to get there, how he/she proposes to achieve these things, identifying what support would be needed from the organisation, what obstacles/difficulties there might be and how these can be overcome.
As I say, may have just been how you phrased it, but worth bearing in mind...
To deal with the specific promotion opportunities you mention. The first prospective promotion was October 2006, but you requested a move sideways which meant foregoing immediate promotion prospects, which sounds fair enough.
Second one was April 2007. Were there specific objectives identified by you or by your boss at your appraisal in October 2006 to enable you to achieve promotion in April, and did you do so? Was promotion guaranteed if you managed these things or was it something to be discussed?
Next objective was promotion 'during 2007'. Were you told why it had been extended to during 2007 rather than being April? I am assuming you asked at your appraisal when you were told it wouldn't be happening?
Now you have been told there will be no promotions until January 2008, which I'm assuming is a budgetary thing or similar, ie nothing to do with you specifically?
I think the key here is what reasons you were given in April 2007, which I think is the only occasion where you may have a legitimate complaint. If you can let us know what the reasons were, it will be easier to identify whether it was reasonable or not, whether there was any discrimination or not. Can't do the maths but were you pg in April and did they know? I think you may have been just pg but early on?
I think the question 'how can I get my promotion tomorrow' is not an easy one to answer. You should be asking your boss (and us or anyone else) how you can get promotion in 6 months or whatever, what you need to achieve/learn/change. It will be too late to get promotion tomorrow, any decision about that will have been made already, no one is going to decide to promote someone because of one meeting. If you have been told nobody will get any promotion before January, I expect that will stand.
You could use your colleague as a comparator, but difficult without knowing more, and as I say, the reasons you have been given in April are key.
More of relevance is the question 'how can I get promotion and payrise before going on maternity leave'. I think if you (and everyone else) have been told no promotions before January, there is no reason to necessarily think that should change. However there is also no reason why you can't be promoted while you are on maternity leave - your performance can be assessed up to the time you go on leave (when?) quite easily, and if you are suitable for promotion and would be promoted if you are at work, there is no reason you can't be. You will only have been on maternity leave for a short while, so you will have recent work experience/performance to be considered. It might be a bit different if you had been on maternity leave for 9 months, I would expect someone in that position to come back and work for a bit to give both them and the organisation an opportunity to fairly assess them before promotion.
As far as discrimination because you are pregnant is concerned, again difficult to tell. It is possible, but would depend on what you are told at your appraisal tomorrow about what you need to do to achieve promotion at the next opportunity which is January, and obviously how well you do it.
HTH, as you can see I'm asking quite a lot of questions, but if you are able to answer them I might be able to help a bit more.